1. Field of the Invention
The present invention relates in general to a water absorptive composition, and more particularly to an optical fiber cable using a water absorptive member formed by using such a water absorptive composition. 2. Discussion of the Prior Art
There are known water-proofed optical fiber cables, which are classified into two types, depending upon the manner in which optical fiber elements are water-proofed. One of the two types uses a water-proofing compound jelly which fills gaps between a plurality of optical fibers and a non-woven fabric disposed around a bundle of the optical fibers. The optical fibers are arranged around the circumference of a central tension member, and the non-woven fabric is covered by a cable jacket which provides an outer surface of the cable. This type of water-proofed optical fiber cable exhibits excellent water-proofing property, but suffers from low production efficiency and difficulty in handling for connection to another cable. Namely, the compound jelly for water-proofing the optical fibers is usually sticky and tacky, and requires an organic solvent for removal thereof.
The other type of the known water-proofed optical fiber cable is shown in FIG. 7. In this type of cable, a central tension member 11 is covered by a water absorptive layer 12, and a plurality of optical fibers 13 are arranged around the circumference of the water absorptive layer 12. Around the circular array of the optical fibers 13, there is disposed an array of water absorptive spun yarns 14 such that each spun yarn 14 extends parallel to the optical fibers. The array of the spun yarns 14 is wrapped and covered by an outer water absorptive layer 15 made of acrylic water absorptive fibers. The outer water absorptive layer 15 is covered by a cable jacket 16, which is the outermost layer of the cable. This type of water-proofed optical fiber cable is disclosed in laid-open publication No. 62-188109 of unexamined Japanese Patent Application.
However, the water absorptive layer 15 made of a water absorptive acrylic fiber does not have sufficiently high hygroscopic swelling ratio and speed. Therefore, where such a water absorptive layer is used for water-proofing an optical fiber cable of a type having fiber accommodating grooves as shown in FIGS. 8 and 9, additional water absorptive tapes are required. More specifically, the conventional optical fiber cable of FIGS. 8 and 9 has an elongated grooved member 2 in which a tension member 1 is embedded in alignment with the axis of the cable. The grooved member 2 has four grooves 3 formed in the outer surface, so as to extend in the longitudinal or axial direction of the cable, such that the grooves are spaced from each other in the circumferential direction of the grooved member 2. Each groove 3 accommodates a plurality of optical fiber tapes 4, and a water absorptive tape 5, which are superposed on each other to form a stack. The grooved member 2 is covered by a water absorptive layer 6, which in turn is covered by an outermost cable jacket 7. The water absorptive layer 6 made of an acrylic fiber is effective to secure water-proofing between the grooved member 2 and the cable jacket 7. However, since the hygroscopic swelling ratio and speed of the water absorptive layer 6 are not sufficiently high, the layer 6 is not sufficiently capable of protecting the interior of the grooves 3 from exposure to water or aqueous components, whereby the optical fiber tapes 4 may be exposed to water in the grooves 3. To solve this problem, the water absorptive tape 5 having the same width (1.5-3.O mm) as the optical fiber tapes 4 is provided for each stack of the tapes 4 in each groove 3, so that the water absorptive tape 5 absorbs water in the groove 3 and hygroscopically swells between the adjacent tapes 4, to thereby protect the tapes 4 from exposure to the water. However, the insertion of the water absorptive tape 5 in each stack of the optical fiber tapes 4 in each groove 3 is considerably cumbersome and time-consuming, resulting in lowering the production efficiency of the optical fiber cable.
In recent years, the water-proofed optical fiber cables are widely used in various fields, for example, as submarine cables. Where the optical fiber cable using only a water absorptive layer as indicated at 6 in FIGS. 8 and 9 is used as a submarine cable, crazing or deterioration of the cable jacket 7 causes entry of the sea water (containing metal salt) into the grooves 3, which may result in the sea water to run in the grooves over a distance as long as several tens of meters, due to insufficient hygroscopic swelling ratio and speed of the acrylic fiber layer 6 with respect to the sea water.